--ion-parallel-compile=on is now the default in the shell

--ion-parallel-compile=on is now the default in the shell

Parallel ion compilation has been the default in the browser for several
months, but was disabled in the shell without a cli argument.

Unifying the defaults should close off yet another pitfall for
volunteers looking to try their hand at optimizing spidermonkey and make
it harder to break inbound with a change that is green locally. Use
--ion-parallel-compile=off to disable it if you need the old behaviour;
however, if you're seeing bustage with the default configuration you
should probably file a bug.

If you know of any other ways that the browser build is substantially
different from the shell build, please let me know.